U.S. patent number 7,078,833 [Application Number 10/159,217] was granted by the patent office on 2006-07-18 for force motor with increased proportional stroke.
This patent grant is currently assigned to Minebea Co., Ltd.. Invention is credited to Yao Hui Xu.
United States Patent |
7,078,833 |
Xu |
July 18, 2006 |
Force motor with increased proportional stroke
Abstract
The force motor of the present invention controls the local
magnetic field through a uniquely designed mechanical structure of
the internal components. The mechanical structure divides the
magnetic field in the force motor into three sections. The force
produced on the armature by the magnetic field in the first section
increases exponentially as the armature approaches the housing. The
force produced on the armature by the magnetic field in the second
and the third sections, as the armature approaches the housing,
counter balances the rise in the force due to the magnetic field in
the first section. Thus, a flat F-S curve over a long stroke length
is obtained.
Inventors: |
Xu; Yao Hui (Mesa, AZ) |
Assignee: |
Minebea Co., Ltd.
(JP)
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Family
ID: |
29582850 |
Appl.
No.: |
10/159,217 |
Filed: |
May 31, 2002 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20030222534 A1 |
Dec 4, 2003 |
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Current U.S.
Class: |
310/14;
310/12.33; 310/13; 310/15; 310/23; 310/30 |
Current CPC
Class: |
H01F
7/14 (20130101) |
Current International
Class: |
H02K
41/00 (20060101) |
Field of
Search: |
;335/255,220,225,229
;310/261,12-14,51 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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847465 |
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Aug 1952 |
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DE |
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204293 |
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Dec 1986 |
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EP |
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WO 9923674 |
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May 1999 |
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WO |
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Other References
Drawings by IMC Magnetics Corp. (15 sheets) of product that was
offered for sale to Eliott Energy Systems of Stuart, Florida. at
earliest date of Jun. 15, 1999. cited by other.
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Primary Examiner: Schuberg; Darren
Assistant Examiner: Comas; Yahveh
Attorney, Agent or Firm: Townsend and Townsend and Crew
LLP
Claims
I claim:
1. A force motor comprising: a shaped housing having: an internal
wall: a cylindrical extension projecting from the internal wall;
and a concave surface formed on the internal wall; a bobbin mounted
in the shaped housing; a permanent magnet mounted in the bobbin,
the bobbin isolating the magnet from the armature thereby
Preventing contaminants from depositing on the armature; a
cylindrical layer located between the bobbin and the armature, the
cylindrical layer being made from an electric conductor and
attached firmly on the armature, thus dampening the movement of
armature due to vibration or shock; a shaped armature mounted in
the shaped housing; wherein the shape of the armature and the
housing cooperate to produce a flat F-S curve for the force motor,
and wherein the shaped armature comprises: a cylindrical portion; a
conical section, the large end of the conical section being larger
than the cylindrical portion; and a cylindrical face formed at the
junction of the cylindrical portion and the conical section, the
cylindrical face extending from the outer surface of the
cylindrical portion to the tip of the large end of the conical
section, and wherein the cylindrical portion, the conical section
and the cylindrical face at the junction of the cylindrical portion
and the conical section are made from materials with different
magnetic properties.
2. The force motor of claim 1, wherein the internal wall, the
cylindrical extension projecting from the internal wall and the
concave surface formed on the internal wall are made from materials
with different magnetic properties.
3. The force motor of claim 1, further comprising: a shim mounted
on the armature, the shim in cooperation with the cylindrical
extension limiting the length of the stroke for the force
motor.
4. The force motor of claim 1, further comprising; a first section
formed by the internal wall and the cylindrical portion; a second
section formed by the cylindrical face and the cylindrical
extension; and a third section formed by the conical section and
the concave conical surface, wherein a force produced on the
armature by a magnetic field in the first section is
counterbalanced by the force produced on the armature by magnetic
fields in the second section and the third section to produce a
flat F-S curve.
5. The force motor of claim 1, wherein the conductive cylindrical
layer is located in the magnetic field of the permanent magnet so
that any movement due to shock or vibration will induce an
electromotive force in the conductive layer thereby damping the
movement.
6. A force motor comprising: a shaped housing; a shaped armature
mounted in the shaped housing, the shaped of the armature and the
housing cooperating to produce a flat F-S curve for the force
motor; a bobbin mounted in the housing; a permanent magnet mounted
in the bobbin, the bobbin isolating the magent from the armature
thereby preventing contaminants from depositing on the armature;
and a cylindrical layer located between the bobbin and the
armature, the cylindrical layer being made from an electric
conductor and attached firmly on the armature, thus dampening the
movement of the armature due to vibration or shock.
7. The force motor of claim 6, further comprising: a shim mounted
on the armature; and a cylindrical extension formed in the housing,
the shim in cooperation with the cylindrical extension limiting the
length of the stroke for the force motor.
8. The force motor of claim 6, wherein the conductive cylindrical
layer is located in the magnetic field of the permanent magnet so
that any movement due to shock or vibration will induce and
electromotive force in the conductive layer thereby damping the
movement.
9. A force motor comprising: a shaped housing, the shaped housing
having a first conical surface; a shaped armature mounted in the
shaped housing, the shaped armature having a second conical
surface, the angle of the first conical surface and the angle of
the second conical surface being selected to produce a magnetic
field that when combined with the magnetic fields between other
portions of the shaped armature and the shaped housing will result
in a flat F-S curve for the force motor; a bobbin mounted in the
housing; a permanent magnet mounted in the bobbin, the bobbin
isolating the magnet from the armature thereby preventing
contaminants from depositing on the armature; and a cylindrical
layer located between the bobbin and the armature, the cylindrical
layer being made from an electric conductor and attached firmly on
the armature, thus dampening the movement of the armature due to
vibration or shock.
10. The force motor of claim 9, further comprising: a shim mounted
on the armature; and a cylindrical extension formed in the housing,
the shim in cooperation with the cylindrical extension limiting the
length of the stroke for the force motor.
11. The force motor of claim 10, wherein the conductive cylindrical
layer is located in the magnetic field of the permanent magnet so
that any movement due to shock or vibration will induce and
electromotive force in the conductive layer thereby damping the
movement.
12. A force motor comprising: a shaped housing; a bobbin mounted in
the shaped housing; a permanent magnet mounted in the bobbin, the
bobbin isolating the magnet from the armature thereby preventing
contaminants from depositing on the armature; a cylindrical layer
located between the bobbin and the armature, the cylindrical layer
being made from an electric conductor and attached firmly on the
armature, thus dampening the movement of armature due to vibration
or shock; a shaped armature mounted in the shaped housing; wherein
the shape of the armature and the housing cooperate to produce a
flat F-S curve for the force motor, and wherein the shaped armature
comprises: a cylindrical portion; a conical section, the large end
of the conical section being larger than the cylindrical portion;
and a cylindrical face formed at the junction of the cylindrical
portion and the conical section, the cylindrical face extending
from the outer surface of the cylindrical portion to the tip of the
large end of the conical section.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This disclosure relates generally to a linear actuated force motor
that requires low power input and provides a long proportional
stroke. More particularly, this disclosure relates to a technique
to control local magnetic field distribution so as to provide a
long proportional stroke.
2. Description of the Related Art
FIG. 1 shows a cross-sectioned view of a conventional force motor.
A conventional force motor includes a shaft 1 mounted in bearings 2
that are mounted in a housing 3. An armature 4 is mounted on the
shaft. Two springs 5 and 6 are mounted on the shaft with the
armature located between the springs. The springs keep the armature
in the neutral position when no net axial force is being exerted on
the armature. The armature shaft is free to slide on the bearings
in axial directions. A permanent magnet 7 is located at the
periphery of the armature. Two coils 8 and 9, wound in the same
direction are located on each side of the permanent magnet.
The permanent magnet produces a magnetic field B.sub.p. When
energized, the coils produce a magnetic field B.sub.i. Since the
coils are wound in the same direction the magnetic field B.sub.i
produced by the coils is in the same direction as the magnetic
field B.sub.p on one side of the permanent magnet and in the
opposing direction on the other side of the permanent magnet. Thus,
the resultant magnetic field on one side of the permanent magnet is
B.sub.p+B.sub.i and on the other side of the permanent magnet is
B.sub.p-B.sub.i. See FIG. 2. The electrical force produced on the
armature is proportional to the square of the magnetic field and
can be calculated as follows. F=KB.sup.2 Eqn. 1 Where F=electrical
force B=Magnetic flux density K=Constant Using equation 1, the net
force on the armature of a force motor when the coils are energized
can be calculated as follows:
.times..times..times..times..times..times. ##EQU00001## For a
proportional solenoid wherein a coil produces a magnetic field
equal to Bi, the net force on the armature can be calculated using
equation 1 as follows: F.sub.ps=KB.sub.i.sup.2 Eqn. 3 Now if
B.sub.p>B.sub.i then 4B.sub.p>>B.sub.i Therefore
F.sub.fm>>F Thus, by using a permanent magnet, for a given
level of coil energization (i.e. current), the force motor produces
larger net force on the armature. Therefore, for a given force
requirement the force motor can be operated with lower power input
compared to the proportional solenoid. If B.sub.p is assumed to be
constant in equation 2, it is clear the net force is proportional
to the magnetic field produced by the coils. F.sub.fm=CB.sub.i Eqn.
4 where C=4KB.sub.p, assuming B.sub.p=constant Since B.sub.i is
proportional to I where I is the current supplied to the coils,
F.sub.fm is proportional to I i.e. the net force on the armature is
proportional to the current supplied to the coils.
However, B.sub.p can be assumed to be constant only when the
armature is in the neutral position. As the armature moves away
from the neutral position, B.sub.p changes. When the armature
moves, B.sub.p on one side of the armature increases whereas
B.sub.p on the other side of the armature decreases. This results
in a dramatic increase in the net force on the armature. Thus, in a
conventional force motor, the force is proportional to the stroke
only within a small range of the stroke, for example 0.01 to 0.03
inches.
U.S. Pat. No. 5,787,915 describes a conventional force motor having
a permanent magnet and coils. However, it does not teach any means
of providing increased proportional stroke.
U.S. Pat. No. 3,900,822 (the '822 Patent) describes a conventional
proportional solenoid with a conical pole piece on each side of the
bobbin. When the solenoid is energized, the armature is pulled to
one side and enters into the conical pole piece. The conical pole
piece provides a leakage flux path and thereby reduces the increase
in the net force on the armature. The proportional solenoid similar
to that of the '822 Patent requires higher power input compared to
the force motor of the present invention to produce the same amount
of force on the armature.
The use of a conical pole piece as taught by the '822 Patent does
not provide a substantial increase in proportional stroke.
Additionally, when a conical pole piece is used, the
proportionality and the constancy of the net force on the armature
gets worse with increase in current (I) supplied to the coils or
when the plunger position changes.
SUMMARY
None of the above mentioned patents teach a force motor with a long
proportional stroke with a flat force versus stroke characteristic
(F-S curve) and low power input.
The force motor of the present invention overcomes the aforesaid
shortcomings of the prior art by controlling the local magnetic
field through a uniquely designed mechanical configuration of the
internal components. The mechanical configuration divides the
magnetic field in the force motor into three sections. In
operation, as the armature moves in the axial direction towards the
end of the stroke, the force exerted on the armature by a magnetic
field in the first section increases exponentially. At the same
time, the force exerted by the magnetic field in the third section
either has a smaller increase compared to the first section, or
decreases. As the armature moves towards the stop, the amount of
magnetic flux in the second section increases. The direction of
this magnetic field is perpendicular to the armature's direction of
movement and therefore does not produce any force in the direction
of the movement thereby reducing the total force on the armature.
By adjusting the mechanical parameters associated with the three
sections, the net axial force on the armature can be controlled,
thereby providing, for a given power level, a flat force vs. stroke
curve over a long stroke.
It is an object of the present invention to provide a force motor
with low power input to achieve a desired force with a flat F-S
curve and long proportional stroke when compared to a conventional
proportional solenoid. These and other objects are accomplished by
providing a housing and an armature movable along an axial
direction in the housing wherein the shape of the armature and the
housing cooperate to produce a flat F-S curve for the force motor.
The invention further contemplates a method of controlling the
magnetic field in a force motor to obtain a flat F-S curve by
forming a first section having a first magnetic field that produces
a force on the armature that increases as the armature approaches
the housing and forming a second section and a third section in the
force motor. The force on the armature due to the a second magnetic
field in the second section and a third magnetic field in the third
section, as the armature approaches the housing, counter balances
the force on the armature produced by the first magnetic field in
the first section to produce the flat F-S curve.
Also provided is a housing having an internal wall, a cylindrical
extension projecting from the internal wall working as a stop to
limit the armature's movement, and a concave surface formed on the
internal wall. An armature supported by the bearing sits in the
housing. The armature includes a cylindrical portion connected to a
conical section. The shape of the armature and the housing are such
that they cooperate to produce a flat F-S curve for the force
motor.
Further features and advantages will appear more clearly on a
reading of the detailed description, which is given below by way of
example only and with reference to the accompanying drawings
wherein corresponding reference characters on different drawings
indicate corresponding parts.
BRIEF DESCRIPTION OF THE DRAWINGS.
FIG. 1 is a cross-sectional view of a prior art force motor;
FIG. 2 shows a magnetic field produced in the force motor of FIG.
1;
FIG. 3 is a cross-sectional view of the force motor of the present
invention;
FIG. 4 is a cross-sectional view of another embodiment of the force
motor of the present invention;
FIG. 5 is an enlarged view of cooperating mechanical structures of
the force motor shown as detail E in FIG. 3;
FIG. 6 is a conceptual representation of the F-S curve for the
three sections formed by the cooperating sections of FIG. 5;
FIG. 7 shows F-S curves for a conventional force motor of FIG. 1
having a greater slope and F-S curves for the force motor of FIG. 4
which are flat.
FIG. 8 shows F-S curves for the force motor of FIG. 3.
DETAILED DESCRIPTION
FIG. 3 shows a cross-sectional view of the force motor of the
present invention. FIG. 4 shows cross-sectional view of another
embodiment of the force motor of the present invention. Force motor
10 includes a shaft 12 which is slidably mounted in bearings 14 and
16. Armature 18 is firmly mounted on shaft 12. Springs 22 and 24
are mounted along shaft 12, one on each side of armature 18. The
assembly of shaft 12, bearings 14 and 16, armature 18 and springs
22 and 24 is mounted in a housing 26. A bobbin 28 is enclosed
within housing 26 and is located at the periphery of armature 18.
Bobbin 28 forms three compartments. In the center compartment is
located a permanent magnet 32. Bobbin 28 prevents contaminants from
magnet 32 from falling on the armature 18. Coils 34 and 36 are
located one on each side of magnet 32 in the compartments formed by
bobbin 28.
Armature 18 is symmetric around the shaft 12 and includes a base 38
connected to a cylindrical portion 42 (see FIG. 3) which in turn is
connected to a conical section 44 having cylindrical face 62
(formed by a counter-bore. In embodiment of FIG. 3, the large end
of the conical section 44 is larger than the cylindrical portion
42. In the embodiment of FIG. 4 base 38 is connected to conical
section 44 having a cylindrical face 62 which in turn is connected
to cylindrical portion 42. In embodiment of FIG. 4, the large end
of the conical section 44 is larger than the cylindrical portion
42. Armature 18 and housing 26 are all made of a ferro-magnetic
material that form a magnetic circuit. A stainless steel shim 46 is
mounted on cylindrical portion 42 of armature 18. By varying the
thickness of shim 46, the travel of armature 18 along shaft 12 can
be increased or decreased; a thicker shim 46 resulting in a shorter
travel distance. Between bobbin 28 and armature 18, along the
periphery of armature 18, is located a cylindrical copper layer 48
that is firmly attached to the armature 18. Copper layer 48 induces
back EMF to dampen the unexpected movement of the armature caused
by vibration, shock, and acceleration.
An internal wall 56 of housing 26 is shaped to form a stop 52. The
shape of stop 52 cooperates with the shape of armature 18 to
provide control of the magnetic field in the area surrounding the
cooperating shapes. Stop 52 includes a cylindrical extension 54
which projects from internal wall 56 of housing 26. Stop 52 also
has a concave conical surface 58 formed on wall 56. Conical surface
58 corresponds to the conical section 44 on armature 18.
Cylindrical extension 54 corresponds to the cylindrical portion 42
and in cooperation with steel shim 46 determines the maximum stroke
length of armature 18.
When coils 34 and 36 are energized by current I, magnetic field
B.sub.i is produced. Magnetic field B.sub.i interacts with magnetic
field B.sub.p as described previously in reference to the
conventional force motor. The action of these two magnetic fields
combined produces a net force F.sub.fm on armature 18. However, as
compared to the conventional force motor, the force F.sub.fm for a
given I remains constant over a longer stroke length for the
reasons explained below.
Force motor 10 of the present invention has shaped armature 18 and
stop 52. The magnetic field between armature 18 and stop 52 is
divided into three sections. FIG. 5 is the enlarged view of
cooperating mechanical structures of armature 18 and stop 52. Also
shown in FIG. 5 are the three sections formed by the cooperating
mechanical structures. FIG. 6 shows a conceptual representation of
the forces in the three sections formed by the cooperating
mechanical structures.
The first section is the magnetic field .PHI..sub.1 formed between
cylindrical portion 42 and internal wall 56. This is equivalent to
a magnetic field inside a solenoid with flat-faced-armature. The
characteristics of the force produced by this field are essentially
exponential increase when the solenoid is pulled-in towards the
stop (see curve A in FIG. 6).
The second section is the magnetic field .PHI..sub.2 located
between face 62 of conical section 44 on the armature 18 and the
face 64 of cylindrical extension 54. As a greater portion of face
62 slides along face 64, .PHI..sub.2 increases. Since .PHI..sub.2
is perpendicular to the direction of motion of armature 18, it does
not produce any significant force in the direction of motion. Line
B in FIG. 6 is a conceptual representation of the force produced by
.PHI..sub.2, that is about zero all over the stroke length.
The third section is the magnetic field .PHI..sub.3 located between
conical section 44 on armature 18 and the conical face 58 on stop
52. It is equivalent to a force in a conical-faced-armature
solenoid. The characteristics of this force curve produced by
.PHI..sub.3 is that it is flatter than that of the first section.
(See curve C on FIG. 6 for a conceptual representation).
When the armature is pulled-in, the second section of magnetic
field .PHI..sub.2 takes away the magnetic flux from the first
section and the third section. Therefore, the force produced by
.PHI..sub.1 and .PHI..sub.3 is actually reduced due to the increase
of leakage flux in the second section, and the force-stoke curves
produced by the magnetic field of the first section and the third
section drop down (see curve A' and C' on FIG. 6).
The resultant force F.sub.fm exerted on armature 18 of force motor
10 is the sum of the force represented by curve A', B, and C'. i.e.
F.sub.fm=F.sub..PHI.1+F.sub..PHI.2+F.sub..PHI.3 Eqn. 5
Thus, by adjusting the cooperating mechanical structures on
armature 18 and stop 52, for example, by varying the shape, size
and angles of cooperating mechanical elements, a desired
force--stroke characteristics curve can be achieved. Adjustment of
force--stroke characteristics may also be done by use of materials
with different magnetic properties. A flat F-S curve advantageously
allows the use of springs with a smaller spring constant, to have
wide range of control and more precise control.
FIG. 7 shows F-S curves for a conventional force motor such as
shown in FIG. 1 and force motor 10 of the present invention as
shown in FIG. 4 for comparison. FIG. 8 shows the F-S curves for the
embodiment of the force motor 10 shown in FIG. 3. The embodiments
shown in FIG. 3 and FIG. 4 have a flat F-S curve over the stroke
length of 0.0 to 0.065 in. and 0.0 to 0.16 in., respectively while
the conventional force motor only has proportional stroke of 0.0 to
0.025 in. The force motors used to obtain the curves had the same
external dimensions, used a similar magnet, used similar coils and
had the same armature diameter. The only difference between the
motors was the presence of cooperating mechanical structures as
described previously in reference to force motor 10. The F-S curves
for the conventional force motor are the ones with greater slope
and shorter stroke. On the other hand, the F-S curves for the force
motor 10 are very much flat over a greatly longer stroke, the
proportional stroke length being (0.15 inches) six times the
proportional stroke length (0.025 inches) for the conventional
force motor. In FIG. 7, the substantially constant force is between
0.2 and 2 lbs. with a variation of about 0.2 lbs. maximum for any
curve. In FIG. 8, the substantially constant force is 0.4 to 5.5
lbs. with a variation of about 1.5 lbs. for any one curve.
The invention controls the slope of the F-S curve even if the slope
is not driven to zero. As shown in FIG. 8, there may be a slight
slope.
While a preferred embodiment of the invention has been described,
various modifications will be apparent to one skilled in the art in
light of this disclosure and are intended to fall within the scope
of the appended claims. For example, the local magnetic field may
be controlled be varying the shape and size or location of the
mechanical configurations in a different manner than described
here. The local magnetic field control may also be achieved by
using different materials with different magnetic properties.
* * * * *